1. Field of the Invention
This invention relates to tunable Fabry-Perot filters and more specifically to partitioned-cavity tunable Fabry-Perot filter having a reduced sensitivity to the angle of incidence.
2. Description of the Related Art
Tunable Fabry-Perot filters are narrowband optical filters that are used to transmit light within a narrow band of wavelengths. Such filters can be used to tune or calibrate lasers or to add/drop channels in an optical network. In these systems the light is typically controlled to enter the filter as a parallel collimated beam where all optical rays are parallel to each other. In other applications the Fabry-Perot filter is part of an imaging system and can be used to detect objects by tuning to specific signature wavelengths, in particular astronomical systems. In many imaging systems, a low system f-number is desirable to maximize the light gathering capability of the system and to minimize the blur circle. The gathered light is focused to a point in the system such that the filter is illuminated with a cone of light distributed from normal incidence to a maximum angle determined by the f-number.
A Fabry-Perot filter is a special case of an interference filter. A standard interference filter includes a pair of planar, parallel reflectors, typically multilayer dielectric films, surrounding a solid cavity layer. The structure behaves as an optical resonator; wavelengths for which the cavity optical thickness is equal to an integer number of half wavelengths are resonant in the cavity and transmitted. Other wavelengths within the reflective band of the reflectors are reflected. The condition for a transmission band center wavelength is given by
                                                                        m                ⁢                                                                  ⁢                λ                            =                            ⁢                              2                ⁢                nd                ⁢                                                                  ⁢                cos                ⁢                                                                  ⁢                                  Θ                  c                                                                                                                      ⁢              where                                                                          m              =                            ⁢                              an                ⁢                                                                  ⁢                integar                                                                                        n              =                            ⁢                              cavity                ⁢                                                                  ⁢                layer                ⁢                                                                  ⁢                refractive                ⁢                                                                  ⁢                index                                                                                        d              =                            ⁢                              cavity                ⁢                                                                  ⁢                layer                ⁢                                                                  ⁢                thickness                                                                                                        Θ                c                            =                            ⁢                              angle                ⁢                                                                  ⁢                of                ⁢                                                                  ⁢                the                ⁢                                                                  ⁢                light                ⁢                                                                  ⁢                within                ⁢                                                                  ⁢                the                ⁢                                                                  ⁢                cavity                ⁢                                                                  ⁢                layer                                                                                        =                            ⁢                                                arcsin                  ⁢                                                                          [                                                            (                                              sin                        ⁢                                                                                                  ⁢                        Θ                                            )                                        /                    n                                    ]                                ⁢                                                                  ⁢                where                ⁢                                                                  ⁢                Θ                ⁢                                                                                          ⁢                                                                                        ⁢                is                ⁢                                                                  ⁢                the                ⁢                                                                  ⁢                angle                ⁢                                                                  ⁢                of                ⁢                                                                  ⁢                incidence                ⁢                                                                  ⁢                in                ⁢                                                                  ⁢                air                                                                                        λ              =                            ⁢                              transmitted                ⁢                                                                  ⁢                wavelength                                                                        (        1        )            
As light impinges on the filter at angles greater than 0° (normal incidence), the passband wavelength shifts to shorter wavelength. Since Θc will be smaller for cavities having a higher refractive index, this angle shift is reduced when higher refractive index materials are used for the cavity layer. This is common in fixed Fabry-Perot filters.
A tunable Fabry-Perot filter is a special case of an interference filter in which the solid cavity layer is replaced with a variable air gap. By adjusting the air gap spacing the resonant wavelength condition is varied, resulting in a tunable passband. The tunable Fabry-Perot filter has a significant dependence of passband on the angle of incidence of light. The refractive index of the airgap cavity is equal to 1.0, resulting in the largest possible value for Θc and consequently the largest wavelength shift as a function of angle of incidence.
As shown in FIG. 1a, the transmission spectrum 10 of a tunable Fabry-Perot filter shifts to shorter wavelengths as the angle of incidence increases from normal incidence (Θ=0°). FIG. 1b shows a transmission spectrum 12 as a function of f-number where the spectral characteristics are calculated by integrating over all rays within the focused cone of light. The f-number is defined as the focal length of the focusing lens divided by the beam diameter at the lens. The lower the f-number the larger the angle of incidence within the cone, hence the larger the wavelength shift and spectral broadening. In this case the filter spectrum shifts to shorter wavelength and broadens, and peak transmission is reduced, the effect increasing with lower f-numbers.
The transmission spectrum shown in FIG. 1b assumes that the incident cone of light is oriented such that the central ray is normal to the filter surface. In actual fact, within an optical system, each point on the image in the focal plane is illuminated by an incident cone of light, each having its central ray at some angle with respect to the normal. Only the central point in the image is illuminated by a normally incident cone of light. Consequently the angle distribution of the light at the optical system focal plane is actually significantly greater than that simply implied by the system f-number due to this “tilt effect”.
One technique for reducing the effect of the angle dependence of a Fabry-Perot filter in an imaging system is to place the filter in front of the focusing lens. In so doing, the filter is illuminated with a parallel collimated beam originating from each point in the object plane (corresponding to a unique point in the image plane). The wavelength shift is still observed because each point in the image is the result of rays entering the system (and hence the filter) at a different angle of incidence. However, the additional spectral broadening that originates from the focusing lens is avoided. In this optical architecture the filter aperture must be as large as that of the optical system, which increases the size, weight and cost of the system. Furthermore, it is not possible to employ a segmented filter array that can wavelength tune each portion of the image independently.
When a Fabry-Perot filter is employed within an optical system, the system is typically designed to have a large f-number to minimize the range of angles employed at the expense light gathering power and blur circle. The “tilt effect” can be significantly reduced by employing a field lens at the original focal plane, creating a telecentric system. Each point in the image is converted to a cone of light with its central ray parallel to the optical system axis. This has the effect of reducing the angle effects of the filter, but the distribution of incident rays is still dictated by the system f-number.
Both techniques to reduce the effect of the angle sensitivity of the tunable Fabry-Perot filters compromise the quality of the imaging system. Neither approach suggests a technique for reducing the angle sensitivity of the tunable Fabry-Perot filter itself while preserving the tunability of the filter.